The ultimate limits to the coherence of long-range acoustic transmissions are imposed by ocean processes, including internal waves, mesoscale variability, interior ocean boundaries (fronts), and bathymetric scattering. An understanding of the effects of these processes on acoustic signals is crucial to the use of acoustic remote sensing methods for a broad range of purposes, including undersea surveillance, ocean acoustic tomography, and large-scale acoustic thermometry. The long-term goals of this research are to enhance our understanding of the ocean processes that ultimately determine the limits of useful long-range acoustic transmissions and to improve our capability to both generate and detect very long-range transmissions. Theoretical considerations suggest that acoustic scattering due to internal-wave-induced sound-speed perturbations will be small at very-low frequencies, i.e., below about 30 Hz, even at multi-megameter ranges. The objective of this research is to understand the frequency dependence of scattering from internal waves and other oceanographic features at multi-megameter ranges.